<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="/js/vue2.js"></script>
</head>
<body>
 <div id="app">
   <input v-model='this.firstvalue' type="text">
   <button>+</button>
   <input v-model='this.lastvalue' type="text">
    <button @click='addvalue'>=</button>
    <input v-model='allvalue' type="text">

 </div>



   <script>
       
    var vm = new Vue({
        el:'#app',
        data:{
           firstvalue:'1',
          lastvalue:'2',
          allvalue:''
        },
      methods: {
          addvalue(){
      this.allvalue=Number(this.firstvalue)+Number(this.lastvalue)
          }
      }
    })
    
    </script>  
</body>
</html>